1. Field of the Invention
The present invention relates to a merged MR head made by notching the first pole piece of the head's write element with a notching layer, and also to forming a notched first pole piece with a first pole piece layer and the notching layer, and then milling a second pole piece layer, a gap layer and the notching layer until side walls ol the second pole piece layer, gap layer and notching layer are contiguous.
2. Description of the Related Art
A write head is typically combined with a magnetoresistive (MR) read head to form a merged MR head, certain elements of which are exposed at an air bearing surface (ABS). The write head comprises first and second pole pieces connected at a back gap that is recessed from the ABS. The first and second pole pieces have first and second pole tips, respectively, which terminate at the ABS. An insulation stack, which comprises a plurality of insulation layers, is sandwiched between the first and second pole pieces, and a coil layer is embedded in the insulation stack. A processing circuit is connected to the coil layer for conducting write current through the coil layer which, in turn, induces write fields in the first and second pole pieces. A nonmagnetic gap layer is sandwiched between the first and second pole tips. Write fields of the first and second pole tips at the ABS fringe across the gap layer. In a magnetic disk drive, a magnetic disk is rotated adjacent to, and a short distance (fly height) from, the ABS so that the write fields magnetize the disk along circular tracks. The written circular tracks then contain information in the form of magnetized segments with fields detectable by the MR read head.
The MR read head includes an MR sensor sandwiched between first and second non-magnetic gap layers. and located at the ABS. The first and second gap layers and the MR sensor are sandwiched between first and second shield layers. In a merged MR head, the second shield layer and the first pole piece are a common layer. The MR sensor detects magnetic fields from the rotating disk by a change in resistance that corresponds to the strength of the fields. A sense current is conducted through the MR sensor, where changed in resistance cause voltage changes that are received by the processing circuitry as readback signals. One or more merged MR heads may be employed in a magnetic disk drive for reading and writing information on circular tracks of a rotating disk.
Good design dictates that the write head writes with a wide track profile, while the read head reads a more narrow track profile in order that the read head not pick up signals from adjacent tracks in the presence of track misregistration. Signals picked up from adjacent tracks result in poor readback performance. The write head is also employed to write servo signals on the magnetic disk, in spaced apart sectors dedicated for servo signals. The disk typically has allocated regions dedicated for the imbedded servo information. The servo signals are read by the read head and employed by servo processing circuitry to maintain the write head on track.
In the prior art, the first pole piece layer of the write head has been notched to improve its servo writing performance. The notching forms a portion of the first pole piece into a pedestal with first and second side walls that align with first and second side walls of the second pole tip. With notching, the fringe field at the gap between the second pole tip and the first pole piece is limited to the width of the second pole tip, which defines the width of tracks written on a disk. This is because the field is captured by the pedestal instead of spreading out laterally to the flat portion of the first pole piece on each side of the second pole tip. Accordingly, tracks the magnetic disk have narrow erase bands. From a servo perspective, narrow erase bands improve the quality of the servo pattern which consists of phase aligned transitions. However, data tracks favor wider erase bands which diminishes interference from adjacent tracks in the presence of track misregistration. Since servoing cannot be sacrificed, there is a strong felt need for a write head that writes good servo tracks, but is better than the prior art at writing data tracks.
Typically, a second pole piece, along with its second pole tip, is constructed by frame-plating it on top of the gap layer. After depositing a seed layer on the gap layer, a photoresist layer is spun on the seed layer, imaged with light, and developed to provide an opening surrounded by a resist wall for plating the second pole piece with its second pole tip. To produce a second pole tip with a narrow track width, the photo-resist layer has to be relatively thin. This relationship, referred to as the "aspect ratio", is the ratio of the thickness of the photoresist layer in the pole tip region to the track width of the second pole tip. Preferably, the aspect ratio should be on the order of three. In other words, for a track width of 1 .mu.m, the thickness of the photoresist in the pole tip region should be about 3 .mu.m. If the photoresist is thicker than this, the side walls of the second pole tip, especially at the base, will not be well formed due to scattering of light as it penetrates the photoresist layer during the imaging step.
A prior art process for notching the first pole piece entails ion beam milling the gap layer and the first pole piece, employing the second pole tip as a mask. According to this prior art process (typified in U.S. Pat. 5,452,164 and U.S. Pat. No. 5,438,747), a full film gap layer is formed on a first pole piece layer, followed by frame plating a second pole piece layer and pole tip on the gap layer. The second pole tip layer is employed as a mask during milling of notches in the second pole tip layer. The direction of milling beam forms an angle to a vertical axis while the workpiece is rotated. The procedure first mills through the gap layer, and next mills the first pole piece layer to form the notches. Since each notch site is directly below a respective side wall of the second pole tip, each notch site is milled for about 180.degree. the rotation, and then is shadowed by the second pole tip, preventing milling for the next 180.degree. of rotation.
In order to account for windage (material consumed by processing), the second pole tip is frame plated, wider than a desired target track width, and thicker than a desired height. During milling of the gap layer to form the write gap, the top and first and second side walls of the plated second pole tip layer are partially consumed. During milling of the first pole piece layer to form the notches, the top and first and second side walls of the second pole tip layer are still further partially consumed. During both milling times, milled material is redeposited on the side walls of the second pole tip. This is removed by angling the milling beam closer to a normal to the side walls. This process, referred to as clean-up, requires extra milling time. Because of the long processing time and large windage of the second pole tip it is difficult to keep the track width and the pole tip height within acceptable limits. When the limits are exceeded, a wafer with literally thousands of magnetic head sites must be discarded. Further, increasing the height of the plated second pole tip layer to account for windage, increases the aforementioned aspect ratios, making it difficult to construct a well-defined second pole tip with a submicron track width. Track widths 1 .mu.m or less are desirable to increase tracks per inch (TPI) written on the disk.
Accordingly, there is a strong felt need to reduce the processing time required for notching, without sacrificing narrow track widths and quality of the write head.